These biscuits are both gluten-free and dairy-free, but deliciously tender on the inside and crisp along the edges. Serve them up for breakfast with butter and jam or alongside a steaming bowl of soup.
Combine flours with xanthan gum and whisk to evenly combine. Using a pastry cutter, cut coconut oil into flour mixture until it becomes crumbly, the size of a small peas.
In a small bowl, beat the egg, then whisk in the coconut milk and sourdough starter. Pour into the flour mixture and mix with a fork until a dough begins to form. Knead the dough with your hands in the bowl until it comes together in a cohesive mass. The dough may be sticky at this point.
Cover the dough and sour 4-12 hours. At first there will be no noticeable tang, then the dough will develop to produce a noticeable sourdough flavor.
Preheat oven to 425°F.
Sprinkle baking powder, baking soda, and salt over the biscuit dough. Knead in for a couple of minutes by folding the dough over itself over and over again.
Once the dry ingredients are incorporated, sprinkle a work surface with tapioca flour. Move the dough to the floured surface. Knead a few times, adding more flour as needed, forming a slightly sticky yet cohesive dough.
Pat the dough out into a circle 1 inch high. Cut the dough into eight triangular biscuits or use a biscuit cutter to cut biscuits as desired.
Carefully transfer biscuits to a cast-iron skillet or baking sheet. The dough will still be fragile so treat it gently. Leave 1 to 2 inches of space between biscuits, as they will expand.
Bake 12-15 minutes, or until lightly brown. These biscuits may not turn as golden brown as traditional wheat biscuits. Remove to cooling rack and cool 5 minutes before serving.